0) Since you plan to use the MiniDSP working as digital crossover and for EQ.

1) Put the Faital Pro 18FH500 into a large'ish SEALED box to get the best group delay, phase control, and room equalization. Dr. Geddes articles on using a few sealed woofers placed around the room might give you ideas for future expansion with additional sealed 18FH500.

2) The controlled directivity and EconoWave threads provide info for what type of waveguide polar pattern, crossed at what frequency will smoothly match the polar response of your Faital Pro 12PR300. BUTT the 12PR300 as close as possible to the waveguide, and carefully select the Xover frequency and slope to have a smooth polar across the full Xover range.

3) Select a 1" compression driver plus waveguide tweeter.
-- I suspect you might select a 1" compression driver with a plastic dome like the DE 250 TN, Faital HF10AK, Beyma CP385Nd.
-- I suspect you might decide to Xover the 12PR300 around 1200Hz to a waveguide with a horizontal polar between 80 and 60 degrees and a 40 degree vertical polar to reduce room floor/ceiling reflections. (The plot I have for the 12PR300 shows 45 degree polar at 1.6Khz) A 90 degree SEOS 15 would probably need to push the Xover down near 1100Hz.
-- I suspect you might decide to use a shallow(< 4") waveguide that physically time aligns the 12PR300 voice coil with the compression driver.

DIPOLE PLAY...
I suspect you would need a large'ish H-frame around the Faital 18FH500 if you want to experiment with dipole bass.
I suspect you will need a modest area, but clever shape baffle to blend a Faital 12PR300 dipole midbass polar to the 18FH500 H-Frame.
I suspect you will need a 4" upper mid dipole on a narrow baffle to carry a smooth dipolar polar pattern above the 12PR300.
I suspect you will need front+rear dome tweeters to maintain the same polar pattern and tone as the other cone drivers

Attached a few examples of short depth horns/waveguides that could physically time align with your 12" after you measure it.

The Daniel Hertz M1 speaker that that you mention is reported to use a 3.8"-3.9" deep 60x40 horn in an effort to have good physical time alignment with the 12" Beyma 12PD80Nd. The Eminence LT250 horn has these specs. Good time alignment is valuable when simple passive Xovers with no delay control are used.

The SEOS 12 waveguide is 3.6"-3.7" deep for 90 x 40 polars, and this shallow depth produces good physical time alignment with 10" and 12" midwoofers. EcoWave designs are well reviewed.

Some designers debate that deeper horns like the H290C @6.6" have better pattern control. There are several deep-frame midbass woofers which time match these deeper horns.

I'm kind of odd in that I like NOT having to make a rigid box, BUT I also do not like the added room signature a dipole adds to the sound. Others really like the enlarged and slightly diffused image that dipoles produce. So I made a super rigid baffle and then use bonded logic and foam to reduce the rearward radiation on the Neo10. The only real dipole range on my 4 way system is the woofers running from approx. 60-250Hz. Below is a sealed subwooofer and above the Neo10 slowly transitioning to more cardioid.

As to the sound, in a large or well treated room they are pretty similar, especially in the midrange. The Neo8S and Neo10 are my favorite mid drivers out there. To my ears, the OEM 70-20 Raal is a tad sweeter and even more resolving than the 140-15. As you can see from the photo, we had both systems set up in the same, large recording studio well away from any boundaries other than the floor. This room is 30ft * 60ft * 15ft tall. I hate to admit it, but something with the mids and highs of the little monitors throw an even wider, deeper soundstage than my baffles. Both sounded the best they ever had and the presense and impact of my system killed the monitors, but the delicacy and ambiance retrieval of the monitor was tops. I'm thinking of upping the my game and swapping in the amorphous cored 140-15 for a trial just to see if it has as big if an impact as Aleksandar claims.

So, my questions to you are, have you ever heard a good dipole whether it was planar or dynamic? Did you enjoy what it sounded like? Who large is your room? Will you be treating your room?

Why not throw together a quick prototype open baffle or swinging nude tester? See if it works for your tastes. If not, buy more wood and make some cabs. I'd certainly plan on running a true tweeter eventually. Maybe wait for a good deal on the Raal 70-10 to pop up.

I'm thinking 18" no higher than 100-150Hz, the 12" mid up to 1KHz absolute highest and EQ the Neo8 to taste from there. In terms of output, the Neo8 will be the weak link in the chain, but I think for a home environment, you will have more than enough SPL capability. We're running our Neo8 down to 450Hz and the sealed, EQ'd woofer is bottoming out before any audible strain on the mid.